It is often desired to dispense a small amount of material to a precise location, such as when dispensing liquid material including solder fluxes and adhesive materials during the fabrication of a printed circuit board. Throughout this application the term "liquid material" or "material" is intended to mean any substance including but not limited to semisolid or highly filled materials such as adhesives and coatings having by volume or weight fifty percent (50%) or above solids, epoxy, silver filled solder paste, thermogreases, etc., and low viscosity materials such as adhesives and coatings having by volume or by weight below fifty percent (50%) solids, solder flux, etc. Contact type dispensers are often used for such applications. These contact type dispensers use syringes with a tip usually located very close to the substrate; i.e. from about 0.005 inches to 0.060 inches away. The liquid material is held within the syringe and is pushed out of the syringe tip. The liquid material contacts the substrate while it is still in contact with the syringe tip. After the liquid material contacts the substrate, the syringe tip is pulled back from the substrate, thereby drawing out a string of material. Upon sufficient retraction of the syringe, the string is broken and the liquid material forms a droplet on the substrate. One problem with the contact type dispenser is that it requires significant movement of the dispenser away from the substrate to separate the liquid material from the syringe, thereby adding to the time and complexity of the dispensing operation. Another problem with the contact type dispensers is that the string of liquid material can break in a manner such that material ends up on the dispenser instead of the substrate or the droplet does not comprise all the dispensed liquid material, but rather a portion of the material separates from the droplet and lands at an undesignated portion of the substrate.
Non-contact dispensers have been developed to overcome the problems associated with the contact type dispenser. In known non-contact type dispensers a reciprocal plunger or piston engages a valve seat that is immersed in the liquid material to be dispensed. In order to dispense a droplet, the plunger is retracted from the valve seat, allowing the liquid material to flow into the area surrounding the valve seat and into an orifice formed in the seat. The plunger is then urged downward into contact with the seat, thereby forcing the liquid material through the orifice. The plunger is accelerated toward the seat to impart sufficient kinetic energy such that a droplet of the liquid material is formed. The droplet breaks away from the end of the dispenser and is propelled downward toward the substrate.
Specific problems arise when dispensing liquid material from known non-contact dispensers. For example, if solder paste is used, the repeated impact of a plunger against the valve seat compacts and "flattens" the solder balls in the paste. The flattened or deformed solder balls cluster and conglomerate, and block or impair the flow of material through the dispenser. In addition, the surface-to-surface contact between the plunger and the seat, when the contacting surfaces are surrounded by solder paste, creates wear due to the abrasive characteristic of the solder paste. Accordingly, it would be desirable to have a non-contact liquid material dispenser which does not cause the liquid material being dispensed to cluster and block the flow of the liquid material and which does not have an abrasive action between the plunger and valve.